Slide rule for computing motor boat performance

ABSTRACT

Disclosed herein is a slide rule device including a base member and a slide member which is slidably carried by the base member. Each member has a pair of logarithmic scales which are located in spaced relation on the front faces thereof and are calibrated to respectively indicate boat gross weight, engine horsepower, boat speed and a boat performance factor which is a function of the boat gross weight, engine horsepower and boat speed and is indicative of boat performance. These scales are arranged in interrelationship on the members so that the performance factor for a boat having a known gross weight, propelled by an outboard motor or stern drive having a known horsepower, and having a known speed can be easily computed by simply moving the slide member relative to the base member to align the appropriate values of the gross weight and engine horsepower scales. Once the performance factor for a boat has been computed or estimated, the predicted speed of the boat, when there is a change in gross weight and/or engine horsepower, can be computed by aligning the appropriate values on the gross weight and engine horsepower scales.

United States Patent Granholm SLIDE RULE FOR COMPUTING MOTOR BOAT PERFORMANCE [75] Inventor: Lars E. Granholm, Waukcgan. 111.

[73] Assignee: Outboard Marine Corporation,

Waukegan. Ill.

[22} Filed: Sept. 16, 1974 [21] Appl. No.: 506,588

Related U.S. Application Data [63] Continuation-impart of Ser. No. 405,041, Oct. 10.

1973. abandoned.

[52] U.S. Cl. 235/70 A; 235/85 [51] Int. Cl. G06G l/02; 606C 3/00 [58] Field of Search 235/70 A, 70 R, 78, 84, 235/85, 89

[56] References Cited UNITED STATES PATENTS 1,581,567 4/1926 Eshbaughu; 235/70 R 2,410,210 10/1946 Goudime-Levkovitsch 235/84 2.443.882 6/1948 Allen 235/70 A 3.091.389 5/1963 Wiken 235/70 R 3.565.335 2/1971 Holden 235/70 R Prinmry E.\'aminerStephen J. Tomsky AIIOHIE), Agent, or FirmMichael, Best & Friedrich 5 7 ABSTRACT Disclosed herein is a slide rule device including a base member and a slide member which is slidably carried by the base member. Each member has a pair of logarithmic scales which are located in spaced relation on the front faces thereof and are calibrated to respectively indicate boat gross weight, engine horsepower, boat speed and a boat performance factor which is a function of the boat gross weight, engine horsepower and boat speed and is indicative of boat performance. These scales are arranged in interrelationship on the members so that the performance factor for a boat having a known gross weight, propelled by an outboard motor or stem drive having a known horsepower, and having a known speed can be easily computed by simply moving the slide member relative to the base member to align the appropriate values of the 1 gross weight and engine horsepower scales. Once the performance factor for a boat has been computed or estimated, the predicted speed of the boat, when there is a change in gross weight and/or engine horsepower. can be computed by aligning the appropriate values on the gross weight and engine horsepower scales.

9 Claims, 4 Drawing Figures SLIDE RULE FOR COMPUTING MOTOR BOAT PERFORMANCE CROSS-REFERENCE TO RELATED APPLICATION I This application is a continuation-in-part of application Ser. No. 405,041, filed Oct. 10, 1973, now abandone'd.

BACKGROUND OF THE INVENTION This invention relates to a slide rule device for computing various performance parameters for motor boats.

From time to time, users of boats propelled by outboard motors, stern drive units, or inboard marine installations desire to know the approximate effect a change in engine horsepower and/or the gross weight of the boat will have on a boat speed. In lieu of an actual trial, the effect can be approximated by computing a performance factor which is the function of the boat gross weight, the engine horsepower and the boat speed. The average person is not knowledgeable of the mathematical equations involved in determining this performance factor. Moreover, complicated, multiple computations are required, making manual or mental computing of this factor both time consuming and subject to error. Once the boat speed has been determined, it is also desirable to select a propeller which will provide the most efficient boat operation for that speed. Such a selection is usually made by referring to a catalog or the like.

SUMMARY OF THE INVENTION According to the invention, there is provided a simple slide rule device including two relatively movable members having logarithmic scales which are calibrated to respectively indicate boat gross weight, engine horsepower, boat speed and a performance factor indicative of the boat efficiency. The scales are arranged in interrelationship so the performance factor can be easily and quickly computed by simply moving the members relative to each other to align the appropriate values on adjacent scales.

, More specifically, the slide rule device includes a first logarithmic scale on a firsti rnember and having graduated units for indicating boat gross weight, a second logarithmic scale on a second member adjacent the first or gross weight scale and having graduated units for indicating engine horsepower, a third logarithmic scale on either the first or second member and having graduated units for indicating boat speed, and a fourth logarithmic scale on the other member (i.e., the member other than the one on which the boat speed scale is located) adjacent the boat speed scale and having graduated units for indicating a boat performance factor. At least one of the first and second scales includes portions of first and second successive logarithmic cycles of the same length and the other scale includes at least a portion of a logarithmic cycle of the same length as the first and second cycles. The third or boat speed scale and the fourth or boat performance factor scale each include at least a portion of one logarithmic cycle having a length twice that of the logarithmic cycles of the first and second scales. The four scales are located on the members in an interrelated manner such that,

logarithmic cycle of the other scale, the graduations of the third and fourth scales are aligned whereby the performance factor is equal to boat speed in miles per hour when the graduations of one of the first and second 7 scales are aligned with the graduations of the second times the square root of the quotient of boat gross weight in pounds divided by engine horsepower.

Preferably, the two members are generally flat and rectangular with one being a base member in the form of a flattened sleeve and the other being a slide member which is slidably received within the sleeve. The base member includes an elongated opening for viewing a portion of the front face of the slide member. Two of the scales are located on the base member and respectively extend thereon lengthwise along the upper and lower edges of the opening and the other two scales are located on the slide member and respectively extend lengthwise thereon adjacent to the upper and lower edges of the base member opening. Propeller selector charts listing the preferred type of propeller for use on a variety of different boats at different speeds can be provided on the rear faces of the base member and the slide member.

A principal feature of the invention is the provision of a simple slide rule device which is adapted to compute performance parameters for motor boats.

Another principal feature of the invention is the provision of a slide rule device adapted to compute a performance factor for'motor boats, which factor is indicative of boat efficiency.

A further principal feature of the invention is the provision of such a slide rule device including means for selecting the proper propeller for a particular type boat at different speeds.

Other features, aspects, and advantages of the invention will become apparent upon reviewing the following detailed description, the drawing, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front plan view of the slide rule device embodying various of the features of the invention.

FIG. 2 is a sectional view taken along the line designated 2-2 in FIG. 1.-

FIG. 3 is a fragmentary, rear plan view of the slide rule device shown in FIG. 1.

FIG. 4 is a front plan view of the slide rule device shown in FIG. 1 shown with a slide member in a position where the graduations of adjacent scales are aligned.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Before explaining the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawing. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting.

Illustrated in the drawing is a slide rule device 10 in cluding a flat, rectangular base member 12 which has a front face 14 and a rear face 16 and is adapted to slidably receive a flat, rectangular slide member 18 which has a front face 20 and a rear face 22. While other arrangements can be used, in the specific construction illustrated, the base member 12 is in the form ofa flattened sleeve having opposed front and rear side walls 24 and 26 which are spaced apart by a pair of parallel spaced, elongated spacer sections 28 and 30 (See FIG.

2). The spacer sections 28 and 30 provide the necessary spacing between the side walls 24 and 26 for affording slidable movement of the slide member 18 and also serve as guides for the opposite edges 32 and 34 of the slide member 18. Both the base member l2and the slide member 18 preferably are constructed from an opaque, relatively thin material, such as cardboard, plastic, and the like. If desired, the spacers sections 28 and 30 can be omitted.

Provided in the front side wall 24 of the base member J 12 for viewing a portion of the front face 20 of the slide member 18 is a generally rectangular window or open- 'ing 36 including parallel, longitudinally spaced upper "and lower edges 38 and 40. While the upper edge 38 and the lower edge 40 of the opening 36 can be equally spaced along their entire lengths, in the specific construction illustrated, the lower edge 40 includes a cut out portion 42 through which a limited portion of the front face 20 of the slide member 18 can be viewed.

values from 150 to 10,000 pounds. If the slide rule device is to be used for larger boats, the boat gross weight scale 44 can include one or more additional successive logarithmic cycles of the same length.

As used herein, logarithmic scale means a scale having graduations which are spaced in logarithmic relation i.e., the actual distances of the graduations from the origin are proportional to the logarithms of the corresponding graduation numbers rather than to the numbers themselves. As used herein, a logarithmic cycle means each portion of a logarithmic scale where the distance or spacing between successive equivalent graduated units progressively decreases until reaching a point where the same pattern of spacing is repeated. The startof each successive new cycle has a value 10 times that of the start of the preceding cycle. Forexample, referring to FIG. 1., the graduations of the boat gross weight scale 44 having the indicated values of 150 to 1,000 constitute a portion of a first logarithmic cycle and the graduations of theboat gross weight scale 44 having the indicated values of 1000 to 10,000 constitute a second or succeeding logarithmic cycle having the same length as the first logarithmic cycle.

Located on the front face of the slide member 18 and extending thereon adjacent to the upper edge 38 of the base member opening 36 and parallel to the boat gross weight scale 44 is a second logarithmic scale 46 which has calibrated graduations for indicating engine horsepower and increases in value from left to right.

The engine horsepower scale 46 includes portions of at j ZsCaIe' 'QSO' is equal in value "to 10 units of the boati least one logarithmic cycle having the same length as the logarithmic cycles of the boat gross weight scale 44. In the specific embodiment illustrated, the engine horsepower scale 46 includes portions of two successive logarithmic cycles and increasesin value from 20 to 600 horsepower. The spacing between the graduations of the engine horsepower scale 46 having the indicated values of 20to and 100,-to 400 is the same as that for the graduations of the boat gross weight scale 44 having the irldicatedvalues of 200 to 1000 and 1000 to 4000, respectively. Thus, the graduations of the engine horsepower scale 46 can be aligned with the graduations of the boat gross weight scale 44 by moving the slide member 18 relative to the base member 12. In the specific embodiment illustrated, when the graduations of these two scales are aligned along their lengths each calibrated unit of the engine horsepower scale 46 is equal in value to 10 units of the boat gross weight scale 44. When the graduations of the first cycle of the engine horsepower scale 46 are aligned with the graduations of the second cycle of the boat gross weight scale 44, each calibrated unit of the engine horsepower scale 46 is equal in value to 100 units of the boat gross weight scale 44. 1

Located on the front face 20 of the slide member 18 and extending lengthwise thereon adjacent to the lower edge 48 of cut out portion 42 is a third logarithmic scale 50 which has calibrated graduations for indicating boat speed in miles per hour. The boat speed scale includes at least a portion of one logarithmic cycle having a length twice that of.each of the logarithmic cycles of the boat gross weight scale 44 and the engine horsepower scale 46. The boat speed scale 50 increases in value from left to right and includes that portion of the logarithmic cycle encompassing the expected range of speeds for the types of boats with which the slide rule device 10 is to be used. In the specific embodiment illustrated, the boat speed scale 50 includes an entire logarithmic cycle ranging from 10 to 100 miles per hour.

Located on the front face 14 of the base member 12 and extending lengthwise thereon adjacent to the lower edge 48 of the cut out portion 42 and parallel the boat speed scale 50 is a fourth logarithmic scale 52 which has calibrated graduations for indicating a boat performance factor. The boat performance factor scale 52 includes at least a portion of one logarithmic cycle having a length the same as that for the boat speed scale 50, i.e., twice the length of each logarithmic cycle of the boat gross weight scale 44 and the engine horsepower scale 46. The boat performance factor scale 52 increases in value from left to'right and includes that portion of the logarithmic cycle encompassing the expected range of performance factors for the types of boats with which the slide rule device 10 is to be used. In the specific embodiment illustrated, the boat performance factor scale 52 includes a portion of the logarithmic cycle ranging from to 200. The spacing between the graduations of the boat speed scale 50? having the indicated values of 14 to 20 is the same as that for the graduations of the boat performance factor scale 52 having the indicated values of 140 to- 200. Thus, the graduations of the boat speed scale 505 can be aligned with the graduations of the boat performance'factor scale 52 by moving the slide;

member 18 relative to; the base member 12 and, when in 'thisposition, each graduated unit of the boat speedv Ce factor scale '52.

i \ll/ith thls'a'r rar'igement, the scales 44, 46, 50 and 52 are interrelated in a manner so that a performance factor indicative of a boat efficiency can be computed in accordance with the following equation:

I boat gross weight (lbs) Performance factor=boat speed(MPI-I) W As used herein, the boat gross weight is the total weight of the boat, engine, passengers, gasoline, auxiliary equipment and gear, etc. To accomplish this computation, the scales are positioned relative to each other so that the graduations of the boat speed scale 50 are aligned with the graduations of the boat performance factor scale 52 when the graduations of the first logarithmic cycle of the engine horsepower scale 46 are aligned with the graduations of the second logarithmic cycle of the boat gross weight scale 44. For example, referring to FIG. 4, it can be seen that, when the graduation on the engine horsepower scale 44 representing horsepower is aligned with the graduation on the boat gross weight scale 44 representing 2000 pounds, all the remaining corresponding graduations on these two scales are aligned and the graduation on the boat speed scale 50 representing 15 miles per hour is aligned with the graduation on the boat performance factor scale representing 150 with all the remaining corresonding graduations on these latter two scales being aligned.

The scales 44, 46, 50 and 52 can be located on the base member 12 and the slide member 18 in various manners so long as the boat speed scale 50 and the performance factor scale 52 are adjacent each other, the gross weight scale 44 and the engine horsepower scale 46 are adjacent each other, and the above-discussed interrelationship of the scales with respect toalignment of the respective logarithmic cycle graduations thereof is provided. For instance, the engine horsepower scale 46 can be located on the basemember 12 adjacent to the upper edge 38 of the opening with the boat speed scale SObeing located on the base member 12 adjacent to the opening lower edge 40, the gross weight scale 44 located onthe slide member 18 adjacent to the opening upper edge 38, and the performance factor scale 52 located on the slide member 18 adjacent to the opening .lower edge 40. In this case, the opening 35 does not include the cut out portion 42, as illustrated in FIG. 1, but instead the upper edge-38 and the lower edge 40 thereof are equally spaced along their entire lengths. Also, if desired, a second opening can be provided in the rear face 16 of the base member 12 with the boat speed scale being located on the rear face of thebase member 12 or the slide member 18 and the performance factor scale 52 located on the rear face of the other member adjacent the boat speed scale 50. Further, the lengths of the base member 12 and the slide member 18 can be extended and all the scales located along the same adjacent edges of the opening 36 and the slide member 18, in which case the boat speed scale 50 and the boat performance scale 52 arelongitudinally spaced from the boat gross weight scale 44 and the engine horsepower scale 46.

Performance of the motor boat (i.e., one propelled by an outboard motor, a stern drive unit, or an inboard marine installation) can be approximately predicted when the gross weight of the boat and the engine horsepower is known. Many different factors, such as hull design, weight distribution, bottom condition, and proniat'ecl without actual testing. The performance factor calculated in accordance with the above equation is an indication of the efficiency of the boat. From experience, it has been found that the performance factor is normally within the range of about and 180 for pleasure boats and is usually higher than 180 for race boats."

In the embodiment illustrated, the longitudinal dimensiori of the cut out portion 42 generally corresponds to the range of typical performance factors for pleasure boats and race boats. As shown in FIG. 1, indicia 53 can be included in conjunction with the performance factor scale 52 for indicating the normal upper and lower limits of the performance factor for pleasure boats and race boats.

When the boat speed, the gross weight of the boat, and engine horsepower are known, the slide rule device 10 can be used in the following manner to determine what effect a different sized motor and/or change in the boat load will have on boat speed. For example, assume that a boat having a gross weight of 2000 pounds and a 20 horsepower engine travels at a top speed of 17 miles per hour. The performance factor for that boat can be determined with the sliderule device 10 by moving the slide member relative to the base member .12 to align the value 20 on the engine horsepower scale 46 with the value'2000 on the boat gross weight scale 44 (See FIG. 4). The value on the boat performance scale 52 opposite to the value l7 on the boat speed scale 50 is the performance factor for that boat. It the engine is replaced with a 40 horsepower engine, the new top speed of the boat can be predicted by moving the slide member 18 until the value 40 on the engine horsepower scale 46 is aligned with the value 2000 on the boat gross weight scale 44. The boat speed of 24 miles per hour is read on the boat speed scale 50 opposite to the previously determined performance factor 170 on the boat performance factor scale 52.

When the boat speed is unknown, the performance factor for the boat is estimated. An accurate estimation of the performance factor requires some judgment based on experience with the particular type of boat being used. However, a performance factor between 155 and 170 normally can be safely used for pleasure boats without introducing a substantial error. To obtain the predicted boat speed, the slide member 18 is moved relative to the base member 12 until the appropriate values on the gross weight scale 44 and the engine horsepower scale 46 are aligned and the boat speed is read on the boat speed scale 50 opposite to the value on the performance factor scale 52 for the previously estimated performance factor.

Also in accordance with the invention, charts listing the preferred propeller for a variety of different type boats at different speeds are provided on the rear face 16 of the base member 12 and on the rear face 22 of the slide member 18. In a specific construction illustrated, a chart 54 indicating the part number and/or other identification of the preferred type. of propellers for boats propelled by an outboard motor is provided on the rear'face '15 of the base member 12 and a similar chart 56 listing the part number and/or other identification of the preferred typeof propellers for boats propelled by a stern drive unit are provided on the rear face 22 of the slide member 18. As shown in FIG. 2,

both of the charts 54 and 56 list the preferred propeller for a specific type boat and a specific range rule device 10 as described above, the appropriate chart 54 or 56 is referred to select the preferred propeller for the boat at that speed.

Although the slide rule device 10 has been described and illustrated as a rectilinear slide type construction, it will be apparent that the device can be arranged in a well known manner to utilize circular or cylindrical type scales so long as the above-described relationships are maintained between the scales 44, 46, 50 and 52.

Various of the feature of the invention are set forth in the following claims.

I claim:

1. A slide rule device for computing performance of a motor boat comprising a first member, a second member mounted for movement relative to said first member, a first logarithmic scale on said first member and having graduated units for indicating boat gross weight in pounds, a second logarithmic scale on said second member and having graduated units for indicating engine horsepower, said first and second scales being adjacent with said graduated units thereof being alignable by relative movement of said members, a third logarithmic scale on said second member and having calibrated units for indicating boat speed in miles per hour, and a fourth logarithmic scale on said first member and having graduated units for indicating a boat performance factor which is indicative of boat efficiency, said third and fourth scales being adjacent with said graduated units thereof being alignable by relative movement of said members, one of said first and second scales including portions of first and second successive logarithmic cycles of the same length and the other of said first and second scales including a portion of a logarithmic cycle of the same length as said logarithmic cycles of said one scale, said third and fourth scales including a portion of 'a logarithmic cycle having a length twice that of said logarithmic cycles of said first and second scales, and said first, second, third, and fourth scales being interrelated such that, when said graduations of said other of said first and second scales are aligned with said graduations of said second logarithmic cycle of said one scale, said graduations of said third and fourth scales are aligned, whereby said performance factor is equal to boat speed in miles per hour times the square root of the quotient of boat gross weight in pounds divided by engine horsepower.

2. A slide rule device according to claim 1 wherein said first member has a front face including spaced upper and lower edges defining an elongated opening located in said front face thereof, said second member has a front face and is slidably carried by said first member for longitudinal movement relative to said opening, the two scales on said first member being respectively located on the front face thereof adjacent to the upper and lower edges of said opening, and the two scales on said second member being located on the front face thereof and respectively adjacent to the upper and lower edges of said opening.

3. A slide rule device according to claim 2 wherein said first scale is located on said base member and extends lengthwise thereon adjacent to the upper edge of said opening, said second scale is located on said slide member and extends lengthwise thereon adjacent to the lower edge of said opening, and said fourth scale is located on said base member and extends lengthwise thereon adjacent to the lower edge of said opening.

4. A slide rule device according to claim 2 including an elongated cut out portion in said lower edge of said first member having a longitudinal dimension generally corresponding to the normal range of performance factors for pleasure boats and race boats and having a lower edge, said performance factor scale being located adjacent said lower edge of said cut out portion and said boat speed scale communicating only with said out out portion.

5. A slide rule device according to claim 1 wherein said first and second members have rear faces, and means located on one of said rear faces for identifying the appropriate propeller to be used for variety of boat types at different ranges of speed.

6. A slide rule device for computing performance of a motor boat comprising a first member, a second 7 member mounted for movement relative to said first member, a first logarithmic scale on said first member and having calibrated units for indicating boat gross weight in pounds, a second logarithmic scale on said second member and having calibrated units for indicating engine horsepower, said gross weight and engine horsepower scales being adjacent with said calibrated units thereof being alignable by relative movement of said members, a third logarithmic scale on said second member and having calibrated units for indicating boat speed in miles per hour, and a fourth logarithmic scale on said first member and having calibrated units for indicating a boat performance factor which is indicative of boat efficiency, said boat speed and performance factor scales being adjacent with said calibrated units thereof being alignable by relative movement of said members, said first, second, third and fourth scales being oriented such that said performance factor is equal to boat speed in 'miles per hour times the square root of the product of boat gross weight in pounds divided by engine horsepower.

7. A slide rule device according to claim 6 wherein said first member has a front face including spaced upper and lower edges defining an elongated opening located in said front face thereof, said second member has,a front face and is slidably carried by said first member for longitudinal movement relative to said opening,{he two scales on said first member being respectively located on the front face thereof adjacent to the upper and lower edges of said opening, and the two scales on said second member being located on the front face thereof and respectively adjacent to the upper and lower edges of said opening.

8. A slide rule device according to claim 7 including an elongated cut out portion in said lower edge of said first member, having a longitudinal dimension generally corresponding to thenormal range of performance factors for pleasure boats and race boats, and having a lower edge, said performance factor scale being located adjacent said lower edgeof said cut out portion and said boat speed scale communicating only with said cut out portion.

9. A slide rule device according to claim 6 wherein said first and second members have rear faces, and

- means located on one of said rear faces for identifying boat types at different ranges of speed. 

1. A slide rule device for computing performance of a motor boat comprising a first member, a second member mounted for movement relative to said first member, a first logarithmic scale on said first member and having graduated units for indicating boat gross weight in pounds, a second logarithmic scale on said second member and having graduated units for indicating engine horsepower, said first and second scales being adjacent with said graduated units thereof being alignable by relative movement of said members, a third logarithmic scale on said second member and having calibrated units for indicating boat speed in miles per hour, and a fourth logarithmic scale on said first member and having graduated units for indicating a boat performance factor which is indicative of boat efficiency, said third and fourth scales being adjacent with said graduated units thereof being alignable by relative movement of said members, one of said first and second scales including portions of first and second successive logarithmic cycles of the same length and the other of said first and second scales including a portiOn of a logarithmic cycle of the same length as said logarithmic cycles of said one scale, said third and fourth scales including a portion of a logarithmic cycle having a length twice that of said logarithmic cycles of said first and second scales, and said first, second, third, and fourth scales being interrelated such that, when said graduations of said other of said first and second scales are aligned with said graduations of said second logarithmic cycle of said one scale, said graduations of said third and fourth scales are aligned, whereby said performance factor is equal to boat speed in miles per hour times the square root of the quotient of boat gross weight in pounds divided by engine horsepower.
 2. A slide rule device according to claim 1 wherein said first member has a front face including spaced upper and lower edges defining an elongated opening located in said front face thereof, said second member has a front face and is slidably carried by said first member for longitudinal movement relative to said opening, the two scales on said first member being respectively located on the front face thereof adjacent to the upper and lower edges of said opening, and the two scales on said second member being located on the front face thereof and respectively adjacent to the upper and lower edges of said opening.
 3. A slide rule device according to claim 2 wherein said first scale is located on said base member and extends lengthwise thereon adjacent to the upper edge of said opening, said second scale is located on said slide member and extends lengthwise thereon adjacent to the lower edge of said opening, and said fourth scale is located on said base member and extends lengthwise thereon adjacent to the lower edge of said opening.
 4. A slide rule device according to claim 2 including an elongated cut out portion in said lower edge of said first member having a longitudinal dimension generally corresponding to the normal range of performance factors for pleasure boats and race boats and having a lower edge, said performance factor scale being located adjacent said lower edge of said cut out portion and said boat speed scale communicating only with said cut out portion.
 5. A slide rule device according to claim 1 wherein said first and second members have rear faces, and means located on one of said rear faces for identifying the appropriate propeller to be used for variety of boat types at different ranges of speed.
 6. A slide rule device for computing performance of a motor boat comprising a first member, a second member mounted for movement relative to said first member, a first logarithmic scale on said first member and having calibrated units for indicating boat gross weight in pounds, a second logarithmic scale on said second member and having calibrated units for indicating engine horsepower, said gross weight and engine horsepower scales being adjacent with said calibrated units thereof being alignable by relative movement of said members, a third logarithmic scale on said second member and having calibrated units for indicating boat speed in miles per hour, and a fourth logarithmic scale on said first member and having calibrated units for indicating a boat performance factor which is indicative of boat efficiency, said boat speed and performance factor scales being adjacent with said calibrated units thereof being alignable by relative movement of said members, said first, second, third and fourth scales being oriented such that said performance factor is equal to boat speed in miles per hour times the square root of the product of boat gross weight in pounds divided by engine horsepower.
 7. A slide rule device according to claim 6 wherein said first member has a front face including spaced upper and lower edges defining an elongated opening located in said front face thereof, said second member has a front face and is slidably carried by said first member for longitudinal movement relative to said opening, the two scales on said first Member being respectively located on the front face thereof adjacent to the upper and lower edges of said opening, and the two scales on said second member being located on the front face thereof and respectively adjacent to the upper and lower edges of said opening.
 8. A slide rule device according to claim 7 including an elongated cut out portion in said lower edge of said first member, having a longitudinal dimension generally corresponding to the normal range of performance factors for pleasure boats and race boats, and having a lower edge, said performance factor scale being located adjacent said lower edge of said cut out portion and said boat speed scale communicating only with said cut out portion.
 9. A slide rule device according to claim 6 wherein said first and second members have rear faces, and means located on one of said rear faces for identifying the appropriate propeller to be used for a variety of boat types at different ranges of speed. 